Proposals to pay MPs £23,000 a year on top of their salaries to help them avoid embarrassing disclosures over their expenses provoked anger, as campaigners labelled them a "sneaky" move that would prevent the system becoming more transparent.
A proposed block grant would replace the detailed claims MPs can make from the public purse to reimburse them for spending on their second homes.
The plans would bring backbenchers' automatic annual income to more than £84,000 and end the need for them to submit receipts showing how their claims under the Additional Costs Allowance have been spent.
The House of Commons authorities have just lost a three-year court battle to prevent disclosure of details of spending on MPs' second homes under the Freedom of Information (FoI) Act.
When the invoices were finally published last week, they showed the taxpayer bought a mock Tudor gable for John Prescott's home in Hull, that Peter Mandelson spent nearly £3,000 on a shower and that Tony and Cherie Blair were threatened with bailiffs over an unpaid water bill.
The Commons Estimates Committee, chaired by Michael Martin, the Speaker, is looking at a series of options, although the lump sum idea is understood to be the preferred choice.
Norman Baker, the Liberal Democrat MP for Lewes, said that it would be "disgraceful" if MPs attempted to subvert the FoI Act and to undermine the recent court ruling against the Commons.
He said: "It would mean running to the shadows when it comes to accountability and give the green light to spending public money on items that aren't connected to their duties."
Matthew Elliott, chief executive of the TaxPayers' Alliance, said: "This is a sneaky suggestion that is aimed squarely at obstructing hard-won transparency in MPs' expenses. People have a right to see how their representatives spend their taxes."
A spokeswoman for the committee said its report on expenses reform had not been completed.